1. Field of Invention
The invention relates to a fuel cell system which circulates exhaust gas discharged from an oxygen electrode of a fuel cell so as to be recycled.
2. Description of Related Art
A fuel cell system for generating power through an electrochemical reaction between oxidizing gas, i.e., air, and fuel gas, i.e., hydrogen requires humidification of the oxidizing gas to be supplied to the fuel cell so as to obtain a predetermined power generation efficiency. Generally in the fuel cell system, the exhaust gas that contains vapor generated by the electrochemical reaction on the oxygen electrode is circulated to the side to which the oxidizing gas is supplied as disclosed in the publication of JP-A-8-500931.
The aforementioned system adjusts the flow rate of the exhaust gas to be circulated so as to perform appropriate humidification without using a humidifying module at the side to which the oxidizing gas is supplied. The publication of JP-A-2002-343398 discloses the technology in which a bypass passage is formed such that water content within the fuel cell is removed within a short period when the operation of the fuel cell is stopped.
The aforementioned fuel cell system, as shown in FIG. 5, includes an intake pipe A upstream of a compressor that admits the oxidizing gas, an exhaust pipe B that discharges the exhaust gas from the fuel cell stack, and a connecting pipe C that connects those pipes A and B. A circulating valve V2 is provided in the connecting pipe C so as to adjust the flow rate of the exhaust gas. One end of the connecting pipe C is connected to the portion downstream of a pressure regulating valve V1 that regulates the inner pressure of the fuel cell stack. The exhaust gas from the fuel cell stack is admitted into the intake pipe A from the exhaust pipe B in the course of discharging the exhaust gas to the outside via the pressure regulating valve V1, which is used for humidification of the oxidizing gas to be supplied to the fuel cell stack.
The fuel cell system as aforementioned fails to appropriately control the flow rate of the exhaust gas to be circulated, and accordingly to adjust the humidification amount. As one end of a circulating valve V2 is connected to a portion around an outlet of the exhaust gas, the pressure at the inlet of the circulating valve V2 becomes approximately an atmospheric pressure. Accordingly the difference in the pressure between the inlet and the outlet of the circulating valve V2 becomes small, which makes it difficult to execute the appropriate flow rate control. Additionally the exhaust gas after passing through the pressure regulating valve V1 is influenced by the change in the flow rate in accordance with the request with respect to the output of the fuel cell stack, which may cause pressure fluctuation. The flow rate control of the circulating exhaust gas in consideration with the pressure fluctuation requires further complicated control of the circulating valve V2.